1. Field of the Invention
The invention relates to circuits for generating a series of pulses that are synchronized to the phases of a three-phase power source. In the example described here, the invention is applied to control SCR firing in a DC motor control.
2. Description of the Prior Art
DC motors are powered from a three-phase AC power source which provides six individual line-to-line signals that vary by 60.degree. when viewed as time-varying waveforms. Each phase signal can be rectified, and the average of these six rectified signals provides a DC signal to the motor armature. Where the motor includes a field winding, rectified signals are also used to provide direct current to the field winding.
The primary electronic component for rectifying the AC phase signals is the silicon-controlled rectifier (SCR). In a bidirectional DC motor speed control of the armature-current reversing type, two groups of six SCR's are necessary. One group is actuated to generate armature current in a forward direction and a second group is actuated to generate armature current in a reverse direction. To avoid the occurrence of faults that could result from the effects of counter-EMF in DC electrical motors, there must be precise timing of the pulses which fire the SCR's.
The operation of SCR's must be synchronized to respective phase-varying signals derived from the AC power source. The firing of the SCR's is used to control the amount of direct current supplied to the motor armature, or received from the motor armature when it is regenerating. The motor control generates a firing angle command signal which is applicable to all of the AC signals, and this firing angle signal must be synchronized with each individual AC signal. It has been the practice to use a ramp generating circuit to generate a sawtooth waveform at twice the AC line frequency, and to compare the value of that signal with the firing angle command signal. Where the firing angle command signal is detected during a portion of the sawtooth waveform, the time of its occurrence is marked by the generation of a digital signal which becomes a time-synchronized pulse. Using three ramp and pulse generating circuits, the firing angle command can be synchronized to the six AC line-to-line signals AB, AC, BC, BA, CA and CB, which are used when power is supplied by a three-phase AC source.
A technical problem arises in ramp generating circuits due to variations from the nominal or assumed values of capacitors used in such circuits. If the values are different than assumed, SCR firing may not occur at the precise time desired. Unbalanced firing may occur if the value of a capacitor in one ramp generating circuit differs from the value of a capacitor in the other two ramp generating circuits required for use with a three-phase AC source.
A further problem is the requirement for operation of the motor control circuitry at either 50 hertz, which is the power line frequency in many countries outside the United States, or at 60 hertz, which is the line frequency of power systems in the United States. The prior art has typically required connection of jumper wires or a change of resistors to change from 50 hertz to 60 hertz operation. A circuit that would operate automatically with either line frequency would eliminate the jumper-wiring or resistor selection step, and would provide convenience to the manufacturer and user.